The present invention relates to a transmission unit, and particularly to a planetary transmission unit, in which the two meshing gears have a small difference, for example 1, 2, 3, or 4, in the number of the teeth.
A transmission that has found a wide variety of applications is the planetary transmission with a small difference in the number of teeth of its meshing gears. Chinese Patent Application No. CN 85106692A discloses a three- circle planetary transmission device which includes a high-speed shaft and a low-speed shaft. Three driving sheets, which are made integrally, are eccentrically provided on the high speed shaft and their eccentric directions are circumferentially spaced by 120.degree.. Each of the driving sheets defines a driving hole with annular teeth. An external gear is rigidly fitted on the low-speed shaft and positioned in the driving holes of the sheets with its teeth engaging the teeth in the driving holes of all three driving sheets simultaneously.
It has proved difficult to ensure that the teeth of the external gear engage the teeth of the three sheets uniformly and simultaneously due to fit and manufacturing tolerances noise and heat production during the work of above device. Moreover, this device has high maintenance costs because it uses the three-circle driving sheets.
Chinese Patent Application No. CN89 2 13292.2 also discloses a transmission device. However, the device includes a double crank with one driving sheet. This device produces undesirable vibration because of eccentric load.
Generally, only one speed ratio is obtained from the output shaft of a prior planetary transmission device with a small difference in the number of teeth. If two or more speed ratios are required, a more complicated unit is required resulting in more complex structure and increased volume and weight of the device.
Moreover, in the prior transmission, the rolling bearings provided between the mounting holes of the driving sheets and their corresponding eccentric sleeves are positioned by means of screws threaded in the wall of the mounting holes with their tip ends pressing against the outer rings of the bearings. That arrangement has a disadvantage in that the outer ring of the bearing which deforms as a result of the action exerted by the tips of the screws to adversely impact on the smooth rotation and service life of the bearing.
Accordingly, the object of the invention is to overcome the disadvantages of the prior art and to provide a planetary transmission unit which has a simple structure and runs smoothly, and in which the teeth of the corresponding gears can uniformly mesh and the bearing is uniformly and circumferentially acted on by radial forces.
A further object of the invention is to provide a planetary transmission unit which can provide several speed ratios.